534 



INSECTA. 



than two pairs present. They are never present on the head or on 

 the last abdominal segment. They, are least numerous in the aquatic 

 larva? of beetles and Diptera, which have but two stigmata placed at 

 the hind end of the abdomen on a simple or forked tube. There 

 are, however, often two openings on the thorax in addition. Some 

 water-bugs (e.g., Nepa Ranatra, etc.) have at the end of the 

 abdomen two long grooved filaments which lead at their base into 

 two air cavities. Such water-bugs can by this arrangement take up 

 air like the Dipteran larvse, by protruding the respiratory tube on 

 the surface of the water. 



The trachece (fig. 443), which are kept open by the spiral thickening 

 of the chitinou's membrane lining them, are always more or less 



perfectly filled with air, and on 

 that account have usually a silvery 

 shining appearance. Their internal 

 chitinous membrane is produced by 

 an outer delicate and nucleated cell 

 layer, and is thrown off with the 

 external cuticle and renewed at 

 each moult during larval life. The 

 dilatations which are not unfre- 

 quently present in the course of 

 the trachea;, and which, in strong 

 flying insects, as Hymenoptera, 

 Diptera, etc., are enlarged to form 

 air sacs of very considerable size, 

 may with justice be compared to 

 the air sacs of birds. They possess a 

 delicate chitinous membrane, which 

 exhibits no trace of the spiral fibre. 

 They therefore collapse with great ease, and require for their filling 

 special respiratory movements. These are especially noticeable in 

 the relatively clumsy Lamellicorns before their flight. The arrange- 

 ment and distribution of the tracheal system may easily be described 

 by starting with the origin of the principal trunks from the stigmata. 

 Each stigma leads into one (or more) tracheal trunk, which sends out 

 connecting branches to the neighbouring trunks and gives off a tuft 

 of much branched tubes to the viscera. As a rule, there are formed 

 in this way two independent lateral trunks, which communicate by 

 transverse tubes and give off numerous secondary trunks to the 

 internal organs. The finer branches of the secondary tubes are not 



Fig. 443. Tracheal branch with finer 

 twigs (after Laydig). Z, Cellular 

 external wall ; Sp, cuticular lining 

 (spiral fibre). 



